It is well known that pNDPA hydrochloride slurry is unstable. The instability of the pNDPA hydrochloride reaction slurry in the processes employing an alcohol and an aromatic solvent systems is well known to those skilled in the art, and is sometimes referred to as gelation. On occasion, due to localized overheating and possibly other factors, rapid decomposition has been observed in such two-solvent systems during the preparation of pNDPA hydrochloride. The pNDPA hydrochloride slurry decomposes even at room temperature. If kept over a period of time, a gradual darkening of the brick-red mixture is noted, and the viscosity of the reaction mass increases to the point that the recovery of the undecomposed product is no longer practical. For this reason, once the reaction is completed, the reaction mass is neutralized as quickly as possible.
The state of the prior art has been summarized in U.S. Pat. No. 4,034,042. The shortcomings of the previously disclosed processes as related to the instability of the pNDPA hydrochloride reaction mass are discussed therein in some detail. The improvement claimed in U.S. Pat. No. 4,034,042 is that the pNDPA hydrochloride is kept in solution by substantially increasing the ratio of the alcohol, usually methanol, to the aromatic hydrocarbon solvent. However, this solvent system does not stabilize pNDPA hydrochloride, but only moderates its decomposition so that the possibility of uncontrolled gelation is minimized. As a consequence, the reaction solution has to be neutralized as quickly as possible to prevent serious yield losses. As disclosed in U.S. Pat. No. 4,034,042, FIG. 1, the pNDPA hydrochloride in solution is about 50% decomposed in about four hours at 40.degree. C.
It has been discovered that the pNDPA hydrochloride slurry made in accordance with our invention is very stable when higher saturated aliphatic alcohols, substantially immiscible with water, are employed as the process solvent. For example, pNDPA hydrochloride was prepared at temperatures as high as 50.degree. C. without noticeable loss in yield. The type of rapid decomposition leading to gelation, does not occur when solvents of the present process are used.